It may sound like “I am Groot” over and over again to us, but Groot’s few words are actually a full-formed language, as evidenced by Rocket’s translation of his dialogue across the Guardians of the Galaxy movies. Over time, we see Star-Lord and the other Guardians develop the ability to understand him, too. Avengers: Infinity War threw a curveball at how we believed Groot’s language worked, however, when Thor was able to understand him immediately.
There’s a light-hearted explanation for this in the movie itself, of course, with Thor saying that he took Groot as an elective back at school on Asgard. Guardians writer/director James Gunn has now given some extra clarification on how “Grootspeak” works, though. While engaging with fans on Twitter as part of a Guardians Vol. 2 watch party recently, Gunn revealed that those who are close to Groot learn his language almost through a telepathic connection.
https://twitter.com/JamesGunn/status/1253843102873182208
One fan followed this up by asking how this fits in with Thor’s Infinity War joke. Gunn’s take on it is that the God of Thunder was definitely not telling the truth with that elective story. “Being a god of sorts he doesn’t have the same high bar for understanding Groot as the Guardians do,” Gunn argued.
https://twitter.com/JamesGunn/status/1253843956753764353
So, there you have it. Being a literal deity, Thor instantly developed a connection to Groot upon meeting him in Avengers: Infinity War which allowed him to understand the character from the off, whereas the other Guardians had to get to know him to build up this connection. This is a pretty neat explanation, though here’s one question I have: Star-Lord is half-Celestial, which is tantamount to being a God, so why didn’t he understand Groot straight away, too?
Hopefully, we’ll get more from Groot, and maybe even an exploration of his origins, when the gang returns in Guardians of the Galaxy Vol. 3. Remember, we’ve apparently got “Alpha Groot” to look forward to, according to Vin Diesel.